New System Simultaneously Corrects Tool Position for 5-axis Machine Tools

Traditional piecemeal compensation of one axis at a time does not consider axis kinematic relationships and their effect on volumetric accuracy, an ability needed to meet today’s higher cutting accuracy requirements. The multi-axis methodology of volumetric error compensation (VEC) originated in Boeing R&D, St. Louis MO, and uses laser technology from Automated Precision, Inc. (API), Rockville MD.

Executives at MAG recently announced what they claim to be the “industry’s fastest VEC system,” the MAG VEC, which analyzes and corrects positioning errors in all machine-tool axes simultaneously reducing the time needed to determine error compensations from days to hours, and integrating both linear and rotary axes into the tool point compensation process, noted Jim Dallam, MAG’s VEC product manager.

Developed and proven by a government/industrial consortium, multi-axis VEC was developed to improve machining accuracies on large machine tools used to produce today’s large, monolithic and complex-shaped parts. The MAG system received a Defense Manufacturing Excellence Award from the National Center for Advanced Technologies (NCAT) in December 2009. A Boeing official called it a “groundbreaking process” that will dramatically reduce assembly and fitting costs — $100 million a year on large programs like the F-18 or 700 aircraft series.

“It gives you a practical and affordable way to raise a machine’s process capability, typically in less than a day, to meet the tighter accuracies required on new parts and programs in the aerospace industry,” said Dallam. “It’s one thing to hold tight tolerances over short distances along a linear axis, but it’s far more difficult along all arbitrary contours and orientations within a volume encompassing several meters.”

Multi-axis VEC collectively treats all of a machine’s degrees of freedom that affect tool point positioning, unlike conventional calibration methods that sequentially examine machine motion one axis at time. Conventional approaches to volumetric compensation are generally limited to three linear axes and the associated total of 21 potential motion error sources. However, a typical five-axis machine with linear and rotary axes can have 43 potential error sources, not just 21. The multi-axis VEC system compensates for all these.

“Dallam said. “The MAG VEC considers the full interrelated effects from the kinematic stack-up of the machine tool axes. This holistic methodology enables volumetric error compensation for every point orientation and path combination inside the work volume.”

To operate, an NC program positions the Active Target to a cloud of some 200 points representing a series of statistically random multi-axis “poses” within the work envelope. The same NC program is run three times, first with the Active Target at a long tool length, then twice again at a short tool length. The 200 commanded and measured positions from the first two runs are mathematically combined to establish each tool axis vector orientation and the third run gives a measure for repeatability. Automated software processes all pose/point data as simultaneous polynomial equations to determine volumetric compensation based on the kinematic error model of the machine.

The compensation solution is then entered into the control, where “compile cycle” technology integrates the compensations into real-time CNC path control algorithms. The volumetric accuracy compensations work in conjunction with, and on top of, traditional, underlying single axis and cross-axis comps.

Measurements are automated within a single coordinate system using laser tracker technology, a simple metrology tool that does not require extensive training to use. Calibration is performed in just a few hours in a single setup, compared to conventional methods that require multiple setups and several days of time, yet fail to capture volumetric axis interactions.

Boeing, MAG, API and Siemens were members of the industry/government consortium that developed the VEC under the program for Volumetric Accuracy of Large Machine Tools (VALMT). Other participants were the National Center for Manufacturing Science, U.S. Air Force Logistics Center, Naval Foundry and Propeller Center, U.S. Navy Fleet Readiness Center, East, and U.S. Army Anniston Depot. The system was tested and proved out on three large machine tools offering different axis configurations.

MAG
www.mag-ias.com

Linear Feedback Technology (Linear Motion Part 2)

Linear motion is particularly impacted by the choice of feedback.  And for most systems the use of feedback is not an option.  Linear motors, for example, cannot be operated without a feedback device.  And because of the linear motor’s roots in semiconductor manufacturing, the feedback is usually a high resolution linear tape scale.

How much feedback resolution is enough?  Most of the time more resolution is better.  But there is an element of control theory that says if the feedback resolution is ten times greater than the position accuracy that you are trying to measure, the control system can become unstable.  The other side effect of extremely high resolution feedback is the tendency to “jitter” because it is responding to tiny variations in the real world, which the control system will then have to contend with.  So spending extra money for high resolution feedback may cause other problems.

Where should the resolution be put?  Obviously, if you are using a rotary servo motor, just use the feedback on the motor as the linear position reference.  This works when the required resolution is not very high because in all mechanically linked systems, there is lost motion called backlash between the motor and load.  But most motion controllers and many indexing drives contain dual feedback loops, so using an external feedback sensor will produce great benefit in accuracy and repeatability.

The big benefit in using linear feedback is the elimination of mechanical error as part of the control system.  On a project I did a few years ago we were evaluating a special grinding machine that had a 13 foot long lead screw in it.  The customer know the lead screw had wear and error in it, and that was part of the problem that needed to be addressed in rehabilitating the machine.  Instead of replacing or re machining the lead screw, we specified an external linear tape scale feedback.  The results were fantastic.  Accuracy and repeatability were phenomenal and combined with an integrated servomotor system,  led to a 300% increase inthroughput for the customer.  Backlash? What Backlash?

How much distance do we need to sense?  Some linear motors like piezo-electrics  and voice coil motors have very limited stroke lengths.  Similarly, different feedback technologies have scalability parameters such as sensing airgap and length requirements are considered.  Some feedbacks work in the range of 2 to 6 inches in overall stroke length, some are capable of 3 feet, some up to hundreds of meters.

The exception is the stepping motor and leadscrew combination which can be operated without feedback on the assumption that the load is not varying dramatically.  But even the leadscrew and stepping motor needs feedback when the load varies.  Current detection can be used to determine if the motor has stalled, but doesn’t necessarily give you the opportunity to recover position without an external source.  So the extra cost of external feedback is a judgement call based on the accuracy requirement and how “robust” the system needs to be.

The variety of types of linear feedback are equally challenging, and as with most things, must be considered based on cost and performance.  The most popular feedbacks are linear tapescale systems that use reflected infrared beams that are interpolated to achieve very high accuracy.  The classic linear feedback from the machine tool era is the glass scale which uses through beam optics and a grating embedded in glass to product the linear position information.  Check out companies like Renishaw, Heidenhahn and others for details. Information on  Heidenhahn’s latest innovation is featured on the Project Mechatronics website.

Over the last few years there have been a number of magnetic solutions where a magnetized linear scale is interpolated by taking the sinusoidal waveforms produced by Hall sensors or inductors, and digitizing the results.  Integrated circuitry combining Hall effect arrays and functional support to linearize output are now the prevailing state of the art.  Check out NewScale Technologies Tracker product for details on their new offering.